JPH04167624A - Code conversion method - Google Patents

Abstract

PURPOSE:To reduce a bit rate and a communication cost by performing the code conversion of an EFM modulation signal. CONSTITUTION:The EFM modulation signal comprised of a mark and a space interval is converted to a variable length code of bit length less than that of the signal, and also, a bit string identifiable from a code string converted at a time when the number of converted codes arrives at a prescribed number is inserted. In other words, as for the EFM demodulation signal comprised of the mark off one bit length and the space interval, three and four bit length intervals are converted to specific two bit codes, respectively, and five to seven bit length intervals to specific three bit codes, respectively, and eight to eleven bit length intervals to specific five bit codes, respectively. Thereby, it is possible to reduce mean bit length, and to shorten transmission time, and to reduce the communication cost.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention provides a code conversion method, in particular, a code conversion method for converting an EFM modulated signal consisting of marks and space sections of 3 to 11 bits in length.
This invention relates to a code conversion method for converting into different signals.

(Prior Art) Conventionally, an EFM modulation signal consisting of a mark and a space section of 3 to 11 bits long is used for compact discs (CDs).
) is used for recording. This signal is created by adding a synchronization signal to the NRZ original signal consisting of subcode data, digital audio data, and parity data.
It performs M modulation, and consists of mark and space strings consisting of 3T to IIT, with the unit bit length being . Figure 2 shows an example of the signal, where 21 is 4.32 of the unit bit length.
It is an 18 MHz signal, and 22 is an EFM modulation signal. E
The length of the mark or base of the FM modulation signal is 3 to 11 times longer than the unit bit length. Therefore, in order to digitally transmit this signal, 4.3218 Mb/
A transmission line with a transmission capacity of S is required. However, if you try to transmit this signal using, for example, the IN51500, the IN51500 will have 192 bits (24 bits).
Byte) is regarded as one frame, and 8000 frames are transmitted per second, 192 x 8000 = 1.53
Since it has a transmission capacity of 6 [Mb/S], 4.328/1.536=2.8, which means that it takes about 2.8 times as long as real time.

Therefore, when transmitting an EFM modulated CD signal through IN51500, communication cost increases, and
Even when considering the case where the transmitted signal is recorded on a recording medium such as a tape or an optical disk, this is not preferable from the viewpoint of memory capacity.

(Problem to be solved by the invention) The present invention has been made in view of the above-mentioned circumstances, and
The purpose of this invention is to reduce the bit rate and communication cost by converting the code of the M-modulated signal, and to utilize the CD player and CD signal processing IC.

(Means for Solving the Problems) The present invention provides an EFM consisting of marks and space sections.
It is characterized by converting a modulated signal into a variable length code with a smaller bit length, and inserting a bit string that can be identified as the converted code string when the number of converted codes reaches a predetermined number. This is a code conversion method.

For an EFM modulated signal consisting of mark and space sections with a length of 3 to 11 bits, a specific 2-bit code is used for the 3- and 4-bit length sections, and a specific 2-bit code is used for the 5- to 7-bit length sections. , each with a specific 3-bit code,
For 8 to 11 bit length intervals, each is converted to a specific 5 bit code, and the number of converted codes is
When a predetermined number is reached, a bit string that can be identified as a converted code string can be inserted.

(Function) The present invention considers the average probability of appearance of mark and space sections of an EFM modulated CD signal, and for example, for 3 and 4 bit length sections, 5 bits are added to each specific 2 bit code.
The average bit length is reduced by converting the .about.7-bit length section into a specific 3-bit code, and the 8-11-bit length section into a specific 5-bit code.

In addition, when the number of converted consecutive codes reaches a predetermined number, by inserting a bit string that can be distinguished from the converted code string, even if an error occurs in the code due to an error, the inserted After recognizing the bit string,
The effect of the error can be nullified.

(Example) FIG. 3 shows an example in which the statistical frequency of appearance of marks and spaces between 3T and IIT in an EFM modulated CD signal was actually measured.

As shown in the figure, short bit lengths appear more frequently. However, since LIT marks and spaces are used as frame sync signals and occur regularly, they appear more frequently than 10T marks and spaces. Based on the actual measurement example of appearance frequency shown in Figure 3, marks and spaces from 3T to 11T were considered as nine events from 3 to 11, and the probability of each was calculated.
This is the value in the appearance probability column in the figure.

Encoding is performed based on this probability of occurrence. This encoding method is known as a Huffman code. This encoding method will be explained with reference to FIG. The nine events numbered 3 to 11 are arranged in descending order of probability of occurrence, divided into groups so that the sum of the probabilities of occurrence is approximately divided into two, and one group is assigned O5 and the other group is assigned 1.

In this example, the first division assigns O to events 3 and 4 as one group, and assigns 1 to the other events. Next, the second and subsequent divisions of small grouping are performed so that the sum of the occurrence probabilities is roughly divided into two between each grouped event, and similarly, one of the small groups has 0. Assign 1 to the other. The second split is event 3
and 4, and assign 0 to the former and 1 to the latter. Furthermore, for the group after event 5, 0 is assigned to events 5 and 6, and 1 is assigned to the other events. Thereafter, this procedure is repeated and a unique code is assigned to each event. The code generated in this way becomes a variable length code as shown in FIG.

It is clear that even when the codes obtained in this manner are consecutive, the breaks in the codes are uniquely determined. Therefore, the code having a length of 2 to 5 bits generated in FIG. 1 can be used in place of the EFM modulated CD signal, and can also be transmitted. On the receiving side, by using a conversion table corresponding to the codes shown in FIG. 1, it is possible to convert the mark and space strings of the original EFM modulated CD signal.

However, if an error occurs in the newly generated code due to a transmission error, it may be impossible to convert it to the original 3T to 11T mark and space section even if a conversion table is used. .

To prevent this, another code having a length sufficient to be distinguishable from a 2- to 5-bit code is inserted after 100 newly generated codes have been transmitted, for example. Such a code may, for example, be 2 of the longest 5 bits of the new code
It can be configured by giving a bit length that is more than twice as long. In this embodiment, a 20-bit length consisting of 10 consecutive 0's and 10 consecutive 1's is given as an insertion code.

Calculating from the appearance probability shown in FIG. 1, the average mark and space interval of the EFM modulated CD signal is 4.72T. On the other hand, the average bit length of the newly generated code is 100
Even if you calculate by inserting the insertion signal of each break, it is 2.96
Bit length. Therefore, 2.96/4°72=0.627, and as a result of code conversion, the transmission bit rate can be reduced by about 37%.

(Effects of the Invention) As is clear from the above explanation, according to the present invention, the EF
By converting the code of the M-modulated signal, the average bit length can be reduced, and when transmitting this, the transmission time can be shortened, reducing communication costs, and when storing it, it is possible to reduce the storage capacity. It has the effect of

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of a code conversion method for explaining an embodiment of the present invention, and FIGS. 2 and 3 are explanatory diagrams of an EFM modulation signal. 21... Signal of unit bit length, 22... EFM modulation signal.

Claims (1)

[Claims] An EFM modulated signal consisting of marks and space sections is converted into a variable length code with a smaller bit length, and when the number of converted codes reaches a predetermined number, it is identified as a converted code string. A code conversion method characterized by inserting possible bit strings.